Ink jet printing is a non-impact method for producing images by the deposition of in droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with dispersants or stablizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Water-based pigmented inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process. Because of their nature, pigments are generally considered to be insoluble species. However, some pigments exhibit particle growth by ripening in the ink.
U.S. Pat. No. 5,750,323 related to a process for making a solid particle dispersion by using a second compound that is structurally similar to a primary compound. The second compound is co-milled with the first and the resulting dispersion is resistant to particle ripening.
U.S. Pat. No. 5,738,716 related to the use of Pigment Yellow 74 in an ink jet ink which has a desirable hue and color density. This pigment has the following structure: ##STR2## However, there is a problem using the pigment in an ink jet ink in that it ripens readily in typical aqueous based pigmented ink formulations. This limits its utility because as the particle size grows to unacceptable levels, problems arise related to jettability, the ability to eject ink from the print head, color gamut, some hue shift and density loss.
It is an object of this invention to provide a method of making an ink jet ink using Pigment Yellow 74 without problems induced by particle ripening in the ink.